Assuming that you are using Verizon, you probably need to use mail.airbridge.net as your outgoing server. Most mail servers (like Yahoo in your example) on the internet today won't relay (send mail for people from another host) to prevent spammers from taking over their servers.

Since mail.airbridge.net is only accessible from Verizon, they will allow you to send mail with your return address from another mail server. The internet headers will be a bit funny, but very few people actually look at those headers on every (any?) email message that they receive.

If you have a server that uses SSL or IMAP, then the rules are somewhat different. For the more common POP3/SMTP servers, you can pretty much count on these behaviors.

-PatP